[kh99]

Well maybe I don't understand exactly what you're looking for. There's a template called memberinfo_vistormessage, and in the middle somewhere there's a line:

<div class="visitor_message_body" id="vmessage_text_$message[vmid]">$message[message]</div>

which I believe is what puts the actual message content in there. If it contains an image then it will be an HTML img tag at that point. I guess if you want to find a hook to put a plugin to change it before it gets to that point you'd have to go to the code. It looks like class_profileblock.php and class_visitormessage.php (both in "includes") are involved. If this doesn't help maybe you can clarify and I'll let someone else take a shot at answering.